Nuclear magnetic resonance (NMR) is a phenomenon exhibited by a select group of atomic nuclei and is based upon the existence of nuclear magnetic moments in these nuclei (termed "NMR active" nuclei). When an NMR active nucleus is placed in a strong, uniform and steady magnetic field, it precesses at a natural resonance frequency known as a Larmor frequency, which is characteristic of each nuclear type and is dependent on the applied field strength in the location of the nucleus. Typical NMR active nuclei include .sup.1 H (protons), .sup.13 C, .sup.19 F and .sup.31 P. The resonant frequencies of the nuclei can be observed by monitoring with a radio frequency (RF) receiver the transverse magnetization which results after a strong RF pulse. It is common practice to convert the measured signal to a frequency spectrum by means of Fourier transformation.
Although identical nuclei have the same frequency dependence upon the magnetic field, differences in the chemical environment of each nucleus can modify the applied magnetic field in the local vicinity of the nucleus, so that nuclei in the same sample do not experience the same net magnetic field. The differences in the local magnetic field result in spectral shifts in the Larmor frequencies between two such chemically non-equivalent nuclei, called "chemical shifts". These chemical shifts are interesting in that they reveal information regarding the number and placement of the atoms in a molecule and in the positioning of adjacent molecules with respect to each other in a compound.
Unfortunately, it is not always possible to interpret the frequency spectra produced by the chemical shifts because of other interfering and dominant interactions. This is particularly true in NMR spectroscopy of solids. In liquid NMR spectroscopy the rapid motion of the liquid molecules tends to isolate the nuclei and separate the nuclear interactions, so that it is easier to distinguish separate nuclei in the final output. In solid state NMR, there are many interactions between the molecules which obscure the output. For example, the magnetic moments in neighboring nuclei perturb each other, resulting in interactions called dipole-dipole couplings. These couplings tend to broaden the characteristic resonance peaks and obscure the "fine" resonant structure produced by the chemical shifts. An additional problem found in solids, which is not present in liquids, is that the orientation of the solid molecules is relatively fixed with respect to the applied Zeeman field and, accordingly, the chemical shifts are anisotropic, in that a component of the resonant frequency depends on the physical orientation of the molecules with respect to the applied field.
Therefore, it is essential to suppress some interactions over others to obtain a meaningful output. This is usually done by perturbing the system at selected frequencies to cause unwanted interactions to cancel or average to a reduced amplitude. For example, in solids, the aforementioned chemical shift anisotropy is usually greatly reduced by orienting the solid sample at the "magic angle" (54.degree. 44') with respect to the applied Zeeman field and physically rotating the solid at a relatively rapid rate causing the anisotropic field components to average to zero. This technique is called Magic Angle Sample Spinning (MASS).
Similarly, by well-known techniques, it is possible to reduce the unwanted spin-spin interactions by irradiating the nuclei with additional pulses of RF energy at or near the Larmor frequencies. By properly selecting various orientations and phases of the RF pulses, the polarization of the perturbing nuclear spin systems in neighboring groups can be changed, effectively averaging out the spin interactions so that the contribution to the final output is greatly diminished. For example, such known RF sequences include the so called WAHUHA sequence described in detail in U.S. Pat. No. 3,530,374; the so-called MREV-8 sequence described in articles by P. Mansfield, Journal of Physical Chemistry, V. 4, p. 1444 (1971) and W. K. Rhim, D. D. Elleman and R. W. Vaughan, Journal of Chemical Physics, v. 58, p. 1772 (1972); and a so called BR-24 sequence described in an article by D. P. Burum and W. K. Rhim, Journal of Chemical Physics, v. 71, p. 944 (1979).
In other known techniques, this latter pulsing technique is combined with the aforementioned sample spinning in a technique called CRAMPS (Combined Rotation And Multiple Pulse Spectroscopy).
Although the aforementioned RF pulse sequences are effective in eliminating dipolar coupling, in practice, they are difficult to implement since they generally require special instrumentation and a high degree of technical skill. In particular, the methods are highly susceptible to interference due to inhomogeneous RF fields, pulse imperfections and transmitter misadjustments. Consequently, very precise and sophisticated NMR instruments must be used and great care must be taken to properly adjust the instruments during use.
Accordingly, it is an object of the present invention to provide a method for increasing the resolution of an NMR solid-state spectrum.
It is another object of the present invention to provide a method for increasing the resolution of an NMR solid-state spectrum which does not require special instrumentation or experimental skills.
It is yet another object of the present invention to provide a method for increasing the resolution of an NMR solid-state spectrum which is tolerant of RF field inhomogeneity, pulse imperfections, and transmitter misalignment.
It is a further object of the present invention to provide a method for increasing the resolution of an NMR solid-state spectrum which increases resolution by utilizing a dipolar decoupling multiple pulse RF pulse sequence.
It is yet a further object of the present invention to provide a method for increasing the resolution of an NMR solid-state spectrum in which a multiple-pulse RF pulse sequence time averages to zero the nuclear evolution due to homonuclear dipolar couplings.